Fluids, hydrocarbons for example, are transported from place to place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in a pipeline particularly when a fluid is bought or sold. That transaction, or “custody transfer,” demands a flow meter of particular accuracy. Custody transfer can occur at a fluid measurement station. Measurement stations may include key components, such as the flow meter or measurement device, associated pipes and valves, and electrical controls. The custody transfer flow meter may be a turbine meter, a positive displacement meter, an ultrasonic meter, a coriolis meter, a vortex meter, or another measuring device.
Turbine flow meters measure fluid by measuring the rate of rotation of a turbine disposed in the fluid stream. The fluid stream flowing through the flow meter impinges upon the blades of the turbine causing the blades to rotate. The angular velocity of the turbine rotor is proportional to the velocity of the fluid stream.
The turbine flow meter can include an electromagnetic sensor comprising a permanent magnet and a coil. As each turbine blade, or other flow indicator feature (e.g., paramagnetic button mounted in a rim around the blades) of the turbine, moves into close proximity with the sensor, the magnetic field of the sensor is deflected, producing a voltage in the sensor's coil. Thus, the sensor generates a series of voltage pulses whose frequency corresponds to the rate of rotation of the turbine and the velocity of the fluid stream through the meter.
Some turbine flow meters include more than one electromagnetic sensor. In such meters the signals from the different electromagnetic sensors may be compared to one another to provide validation of proper meter operation. Various attributes of the sensor output signals, including for example frequency, phase, etc., may be compared in a validation process. The positions of the sensors relative to other meter components (e.g., the axis of the turbine rotor) may affect the sensor output signals, and consequently affect the operation of the meter.